EP2184302B1 - Blow moulded milk bottles - Google Patents

Abstract

This invention discloses blow moulded bottles for milk produced with a bimodal high density polyethylene (HDPE) resin.

Description

• The present invention relates to a process for preparing blow moulded bottles with bimodal high density polyethylene resins.
• It is an aim of the present invention to produce blow moulded bottles for milk that have high environmental stress crack resistance.
• It is yet a further aim of the present invention to prepare blow moulded bottles for milk that have good organoleptical and food contact properties because they have a very little content of volatile organic compounds (VOC).
• A process for preparing blow moulded bottles for milk with a bimodal high density polyethylene (HOPE) resin produced with a catalyst system comprising a bisindenyl-based catalyst component and wherein said HOPE resin has a density, measured following the method of standard test ASTM 1505 at a temperature of 23 °C, of from 0.945 to 0.965g/cm³, a melt index MI2, measured following the method of standard test ASTM D 1238 at a temperature of 190 °C and under a load of 2.16 kg, of from 1 to 10 dg/min, and a molecular weight distribution, defined by the polydispersity index D that is the ratio Mw/Mn of the weight average molecular weight Mw over the number average molecular weight Mn, of at least 3.
• The bimodal HDPE resin can be prepared from a physical blend or from a chemical blend. The chemical blend can result for example from a single catalysts system used in a double loop reactor wherein the loops are operated under different polymerisation conditions or from two or more catalyst systems used in a single or in a double loop reactor.
• When a double loop reactor is used, it can be operated under various modes:
• hydrogen split wherein different concentrations of hydrogen are used in the different reactors in order to produce a low molecular weight fraction in a reactor and wherein the polydispersity is broadened in the other reactor;
• comonomer split wherein different comonomer concentrations are used in the different reactors in order to produce a low comonomer concentration in a reactor and a high comonomer concentration in the other reactor;
• comonomer/hydrogen split wherein a high molecular weight and high comonomer concentration is produced in one reactor and a low molecular weight, low comonomer concentratrion is produced in the second reactor. In the direct configuration, the high comonomer concentration is produced in the first reactor and vice versa, in the inverse configuration, the low comonomer concentration is produced in the first reactor.
• The first mode, in direct configuration, is preferred in the present invention.
• Preferably, the bimodal HDPE resin is prepared with a catalyst system based on a bridged bisindenyl catalyst component. The catalyst component is of general formula I
  
          R"(Ind)₂MQ₂     (I)
  
  wherein (Ind) is an indenyl or an hydrogenated indenyl, substituted or unsubstituted, R" is a structural bridge between the two indenyls to impart stereorigidity that comprises a C₁-C₄ alkylene radical, a dialkyl germanium or silicon or siloxane, or a alkyl phosphine or amine radical, which bridge is substituted or unsubstituted; Q is a hydrocarbyl radical having from 1 to 20 carbon atoms or a halogen, and M is a transition metal Group 4 of the Periodic Table or vanadium.
• Each indenyl or hydrogenated indenyl compound may be substituted in the same way or differently from one another at one or more positions in the cyclopentadienyl ring or in the cyclohexenyl ring and the bridge.
• Each substituent on the indenyl may be independently chosen from those of formula XR_v in which X is chosen from Group 14 of the Periodic Table, oxygen and nitrogen and each R is the same or different and chosen from hydrogen or hydrocarbyl of from 1 to 20 carbon atoms and v+1 is the valence of X. X is preferably C. If the cyclopentadienyl ring is substituted, its substituent groups must not be so bulky as to affect coordination of the olefin monomer to the metal M. Substituents on the cyclopentadienyl ring preferably have R as hydrogen or CH₃. More preferably, at least one and most preferably both cyclopentadienyl rings are unsubstituted.
• In a particularly preferred embodiment, both indenyls are unsubstituted, and most preferably they are unsubstituted hydrogenated indenyls. Most preferably it is isopropyledenebis(tetrahydroindenyl) zirconium dichloride.
• The active catalyst system used for polymerising ethylene comprises the above-described catalyst component and a suitable activating agent having an ionising action.
• Suitable activating agents are well known in the art: they include aluminium alkyls aluminoxane or boron-based compounds.
• Optionally, the catalyst component can be supported on a support.
• This catalyst system is preferably used in a liquid full double loop reactor wherein the loops are operated under different conditions in order to produce a bimodal resin. The double loop reactor can be operated either in direct configuration wherein the high comonomer concentration copolymer is prepared in the first reactor or in inverse configuration wherein the low comonomer concentration homopolymer is prepared in the first reactor.
• The bimodal resins of the present invention have densities of from 0.940 to 0.965 g/cm³, preferably of from 0.945 to 0.955 g/cm³ and more preferably of about 0.950 g/cm³. They have a melt index MI2 of from 1 to 10 dg/min, more preferably of from 1.5 to 8 dg/min, most preferably from 1.5 to 4 dg/min. They have a polydispersity index that is preferably of at least 3, more preferably from 3.0 to 4.0 and most preferably from 3.1 to 3.6. The molecular weights are determined by GPC-DRI. In solution, long-branched polymers assume a more compact configuration than linear chains and their molecular weight can thus be slightly underestimated. Density is measured following the method of standard test ASTM 1505 at a temperature of 23 °C. Melt flow indices MI2 and HLMI are measured following the method of standard test ASTM D 1238 at a temperature of 190 °C and respectively under loads of 2.16 and 21.6 kg. Polydispersity index D is defined as the ratio Mw/Mn of the weight average molecular weight Mw over the number average molecular weight Mn and the molecular weights are determined by gel permeation chromatography (GPC).
List of Figures.
• Figure 1 represents the molecular weight distribution of the resins tested.
• Figure 2 represents the complex viscosity expressed in Pas as a function of frequency expressed in rad/s for several resins.
• Figure 3 represents the flow length FL expressed in mm as a function of injection pressure expressed in bars.
Examples.
• Several resins have been tested in the production of caps and closures for carbonated drinks.
• They were selected as follows.
• Resin R1 is a monomodal high density polyethylene (HDPE) resin prepared with isopropylidene-bis(tetrahydroindenyl) zirconium dichloride.
• Resins R3 to R5 are bimodal HDPE resins prepared with isopropylidene-bis(tetrahydroindenyl) zirconium dichloride (THI) in a double loop reactor in inverse configuration, i.e. wherein the homopolymer is prepared in the first reactor.
• Resin R2, R6 and R9 are bimodal HDPE resins prepared with isopropylidene-bis(tetrahydroindenyl) zirconium dichloride (THI) in a double loop reactor in direct configuration, i.e. wherein the copolymer is prepared in the first reactor.
• Resins R7 and R8 are conventional, commercially available, Ziegler-Natta HDPE resins.
• Their properties are summarised in Table I. TABLE I.

  Resin	Density g/cm3	MI2 dg/min	Mn kDa	Mw kDa	D
  R1*	0.949	1.9	24.3	65.3	2.7
  R2	0.950	1.5	23.3	71.2	3.1
  R3	0.950	1.9	22.8	69.2	3.0
  R4	0.949	1.2	22.8	76.7	3.4
  R5	0.949	1.5	21.8	71.8	3.3
  R6	0.948	1.5	24.0	76.8	3.2
  R7*	0.952	2.1	16.5	102.2	6.3
  R8*	0.952	1.9	18.0	108.4	6.0
  R9	0.950	2.0	25.4	72.9	3.6

  * not according to the invention

• The curves representing the molecular weight distribution for all resins are represented in Figure 1. As expected, the molecular weight distribution of all the resins prepared with a Ziegler-Natta catalyst system are significantly broader than those of all the metallocene-prepared resins. In addition, they include very long chains that are characterised by a high molecular weight fraction above 10⁶ daltons at variance with all the metallocene-prepared resins, both monomodal and bimodal that do not contain very long chains.
• The molecular architecture of the resins has also been investigated and the amount of short chain branching and long chain branching has been evaluated for each resin. All samples were very crystalline.
• The short chain branching content was measured by NMR. The results for all resins are displayed in Table II as well as the nature of the short branches.
• The long chain branching content was determined by the long chain branching index (LCBI) method. The method is described by Schroff R.N. and Mavridis H. in Macromolecules, 32, 8454 (1999) and LCBI is given by empirical formula $$\mathrm{LCBI}\mathrm{=}{{\mathrm{\eta }}_{\mathrm{0}}}^{\mathrm{0.288}}\mathrm{/}\mathrm{1.88}\mathrm{*}\left[\mathrm{\eta }\right]\mathrm{-}\mathrm{1}$$

  

  
  wherein η₀ is the zero shear viscosity expressed in Pa.s and [η] is the intrinsic viscosity in solution expressed in g/mol. This method is more sensitive than the usual Dow Rheological Index (DRI) or NMR methods and is independent of the polydispersity. It was developed for substantially linear polyethylene such as typically obtained in metallocene catalysis and it only requires the measurement of intrinsic viscosity of a dilute polymer solution and the zero shear viscosity. It is equal to zero for linear chains and deviates from zero when long chain branching is present. The intrinsic viscosity values were calculated from the Mark-Houwink relationship that was developed for linear chains and it must be noted that this method only applies to resins having a small content of long chain branching. The zero shear viscosity was obtained by Carreau-Yasada fitting. The results are displayed in Table II and they show that the resins prepared with Ziegler-Natta catalyst systems have no long chain branching and that the bimodal metallocene-prepared resins have the highest level of long chain branching. TABLE II.

  Resin	Nature SCB	SCB content wt%	LCBI
  R1	butyl	0.3	0.72
  R2	butyl	0.5	1.5
  R3	butyl	0.8	0.89
  R4	butyl	0.8	1.4
  R5	butyl	0.9	1.13
  R6	butyl	0.7	1.29
  R7	ethyl	0.6	0
  R8	-	-	0
  R9	butyl	0.7	0.7

• The complex viscosity curves as a function of angular frequency are presented in Figure 2. Plate-plate rheometer data were used because they are more precise and more reliable. It is known in the art that shear thinning or pseudo-plastic behaviour is influenced by the presence of long chain branching or by broadening of the molecular weight distribution. As can be seen from figure 2, the bimodal resins prepared with THI have the most pronounced pseudo-plastic behaviour due to the combined effects of the presence of long chain branching and fairly broad molecular weight distribution.
• The temperature dependence of the viscosity can be described by formula $$\mathrm{n}\left(\mathrm{T}\right)\mathrm{=}{\mathrm{a}}_{\mathrm{T}}\mathrm{*}\mathrm{\eta }\left({\mathrm{T}}_{\mathrm{0}}\right)\mathrm{*}\left(\mathrm{T}\mathrm{*}\mathrm{\rho }\mathrm{/}{\mathrm{T}}_{\mathrm{0}}\mathrm{*}{\mathrm{\rho }}_{\mathrm{0}}\right)$$

  

  
  wherein a_T is the time shift factor, T is the temperature and ρ and ρ₀ are the densities respectively at temperatures T and To. Far from the glass transition temperature, as is the case for the polyethylene of the present invention, the flow activation energy E_a can be derived from the Arrhenius relationship: $${\mathrm{a}}_{\mathrm{T}}\mathrm{=}\exp \left({\mathrm{E}}_{\mathrm{a}}\mathrm{/}\mathrm{R}\mathrm{*}\left(\mathrm{1}\mathrm{/}\mathrm{T}\mathrm{-}\mathrm{1}\mathrm{/}{\mathrm{T}}_{\mathrm{0}}\right)\right)$$

  
• The calculated values are reported in Table III. It was observed that the activation energy of all resins prepared with THI had much higher values of the activation energy than those obtained for the resins prepared with Ziegler-Natta catalyst systems: this is due to the presence of long chain branching. TABLE III.

  Resin	Ea kJ/mole
  R1	44
  R2	44
  R3	55
  R4	35
  R5	57
  R6	27
  R7	27
  R8	-

• The bimodal HDPE of the present invention can be used in applications such as for example,
• in blow moulding for milk bottles;

Claims (7)

1. A process for preparing blow moulded bottles for milk with a bimodal high density polyethylene (HDPE) resin produced with a catalyst system comprising a bisindenyl-based catalyst component and wherein said HDPE resin has a density, measured following the method of standard test ASTM 1505 at a temperature of 23 °C, of from 0.945 to 0.965g/cm³, a melt index MI2, measured following the method of standard test ASTM D 1238 at a temperature of 190 °C and under a load of 2.16 kg, of from 1 to 10 dg/min, and a molecular weight distribution, defined by the polydispersity index D that is the ratio Mw/Mn of the weight average molecular weight Mw over the number average molecular weight Mn, of at least 3.
2. The process of claim 1 wherein the bimodal high density polyethylene (HDPE) resin is produced with a bisindenyl-based catalyst system in a double loop reactor wherein the loops are operated under different polymerisation conditions.
3. The process of claim 2 wherein the HDPE resin is prepared with the bisindenyl-based catalyst system in a double loop reactor in direct configuration.
4. The process of claim 2 wherein the HDPE resin is prepared with the bisindenyl-based catalyst system in a double loop reactor in inverse configuration.
5. The process of any one of claims 1 to 4 wherein the bisindenyl-based catalyst component is an unsubstituted bistetrahydroindenyl component.
6. The process of claim 5 wherein the bisindenyl-based catalyst component is isopropylidenebis(tetrahydroindenyl) zirconium dichloride.
7. Blow moulded bottles for milk obtained by the process according to any one of claims 1 to 6.

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